fuse-overlayfs

An implementation of overlay+shiftfs in FUSE for rootless containers.

# Limitations:
Read-only mode is not supported, so it is always required to specify an
upperdir and a workingdir.

# Usage:

```shell
$ fuse-overlayfs -o lowerdir=lowerdir/a:lowerdir/b,upperdir=up,workdir=workdir merged
```

Specify a different UID/GID mapping:

```shell
$ fuse-overlayfs -o uidmapping=0:10:100:100:10000:2000,gidmapping=0:10:100:100:10000:2000,lowerdir=lowerdir/a:lowerdir/b,upperdir=up,workdir=workdir merged
```
